Electrical coils for generating magnetic fields are widely used for, inter alia, influencing the path of an electron beam in an electron beam lithography machine, an electron microscope or other equipment generating and deploying electron beams for various purposes. Thus, for example, deflecting coils are used in lithography machines to provide controlled deflection of a beam with the object of writing a pattern, such as an integrated circuit format, on a substrate. Focussing coils are similarly used in lithography machines for focussing the beam to produce a desired writing beam spot on the substrate. The operation of such a coil results in ohmic heating of the coil wire and consequently an appreciable output of heat, which leads to expansion of the coil. Variable operation of the coil to generate and terminate the magnetic field and/or to vary the intensity of the field results in constant variation in the heat dissipation of the coil and consequently change in the dimensions of the coil, in particular of the coil winding and the former on which the winding is carried, as well as the magnetic field produced by the winding. Changes in the magnetic field detract from accuracy of beam spot placement on the substrate or in the sensitivity of the beam deflection or focussing and this in turn causes, for example increased tolerances in the pattern written by the beam.
In current practice, variations in the dimensions of the coil due to changes in heat dissipation are minimised by passive solutions, such as constructing the winding former and a potting medium of the winding from materials having low coefficients of thermal expansion, the former and the potting medium having the effect of constraining thermal expansion of the winding. This approach is effective within limits, but there remains scope for further improvement in thermal stabilisation in coils employed for generation of magnetic fields with sensitive control applications.